4 enthalpy

8/10/2019 4 Enthalpy

1/27


2/27

EnthalpyA Combination Property

In the analysis of certain types of processes, particularly inpower generation and refrigeration , we frequently

encounter the combination of properties u + Pv. For the sake of simplicity and convenience, this

combination is defined as a new property, enthalpy, and

given the symbol h


3/27


4/27


In analyzing specific types of processes, we frequentlyencounter certain combinations of thermodynamic

properties, which are therefore also properties of thesubstance undergoing the change of state.

To demonstrate one such situation, let us consider a controlmass undergoing a quasi-equilibrium constant-pressureprocess, as shown in Fig.


5/27


6/27


The work done can be calculated from the relation

Since the pressure is constant,


7/27


Therefore,


8/27


We find that, in this very restricted case, the heat transfer during the process is given in terms of the change in the

quantity U+PV between the initial and final states.

Because all these quantities are thermodynamic properties,

that is, functions only of the state of the system, their combination must also have these same characteristics.


9/27


Therefore, we find it convenient to define a new extensiveproperty, the enthalpy,


10/27

Specific Heat


11/27

Specific Heat

We know from experience that it takes different amountsof energy to raise the temperature of identical masses of

different substances by one degree.

For example, we need about 4.5 kJ of energy to raise the temperature

of 1 kg of iron from 20 to 30C,

whereas it takes about 9 times this energy ( 41.8 kJ to beexact) to raise the temperature of 1 kg of liquid water bythe same amount.


12/27

Specific Heat

Therefore, it is desirable to have a property that will enableus to compare the energy storage capabilities of various

substances.

This property is the specific heat .

The specific heat is defined as the energy required to raisethe temperature of a unit mass of a substance by onedegree.

In general, this energy depends on how the process isexecuted.


13/27

Specific Heat

In thermodynamics, we are interested in two kinds of specific heats:

specific heat at constant volume and

specific heat at constant pressure


14/27

Specific Heat : at constant volume

Physically, the specific heat at constant volume can beviewed as the energy required to raise the temperature of the unit mass of a substance by one degree as the volumeis maintained constant .


15/27



16/27



17/27



18/27

Specific Heat : at constant pressure

The energy required to raise the temperature of the unitmass of a substance by one degree as the pressure is

maintained constant is called its specific heat at constantpressure


19/27


The specific heat at constant pressure is always greater than because at constant pressure the system is allowed

to expand and the energy for this expansion work must alsobe supplied to the system.


20/27



21/27



22/27



23/27


24/27

Relationship between

and


25/27


and


26/27


and


27/27

4 enthalpy

Documents